1. Field of the Invention
The low capacitance audio connector for use on audio cables, relates to the transmission of audio information from a source (typically a guitar or musical instrument) to a sink (typically an audio amplifier) with reduced high frequency rolloff attributable to the capacitance of the connector.
2. Description of the Prior Art
An audio connector for use on an audio cable is intended to allow easy, reliable and rapid connection of an audio cable to a musical instrument or amplifier. Audio cables are fitted with connectors at each of two ends. Typical audio connectors have a coaxial construction, with a central single signal conductor surrounded by a tubular shield or ground conductor. Such connectors have capacitance between the signal conductor and shield or ground conductor. These connectors contribute capacitance to the overall capacitance of the cable assembly. For high impedance audio sources, this capacitance acts to degrade the high frequency content of the signal.
Many applications require low capacitance connectors. For example, test and measurement applications are sometimes at risk of fouled measurements due to high capacitance connectors. As a remedy, innovations have been made in the physical design of connectors. Cook (U.S. Pat. No. 7,387,531, Jun. 17, 2008) discloses a universal coaxial connector, and its features: “For many of these and other types of cable, small changes in capacitance/impedance from the connector can often cause significant changes in return loss measurements for the cable. These and other errors are minimized by various aspects of the connector 2, such as the gripping barrel 12, the drain wire 22 and the conductive disk 24, which alone and/or in combination with other features help to reduce stray and/or parasitic capacitance that could otherwise lead to measurement errors.” The advantages stated are a result of optimized physical design of the connector. Such optimizations can reduce the capacitance of a connector only so much.
The telecommunications connector described in Kjeldahl, et al. (U.S. Pat. No. 6,102,730, Aug. 15, 2000) illustrates clearly a problem that is suggested by Cook, above. The problem is that the size of the connector influences greatly the parasitic capacitance of the connector: The smaller the connector, generally the larger the capacitance. Kjeldahl, et al. states, “ . . . it is a desire that the connector be as small as possible, and this, of course, accentuates the capacitive coupling problem because the required small dimensions result in a small distance between the leads of the connector elements and thus a relatively high capacity between these leads.” Separating the stated dependency between the capacitance of a connector and its physical geometry is highly desirable.
The interelectrode capacitance of an audio connector is typically small, on the order of 15 picofarads. This is negligible for some applications. However, in the case where the audio cable itself is operating under a capacitance reduction scheme, such as a driven shield arrangement, the capacitance of a connector at each end of the cable comprises the majority of the capacitance of the entire assembly.
Whereas driven shield arrangements are well known in the prior art as a method of capacitance mitigation, the prior art ignores the capacitance of connectors in audio applications as being negligible. Therefore one finds the prior art devoid of audio connectors specifically designed to participate in driven shield capacitance reduction methods.
Such a driven shield arrangement as applied to a cable requires three conductors, typically in a triaxial configuration. A center conductor carries the signal of interest. A second conductor is arranged as a shield around the center conductor, separated by a first dielectric. An optional semi-conductive layer situated around the outer surface of the first dielectric helps to reduce noise caused by mechanical motion of the cable's components (not shown in the figures). A third conductor is typically arranged as an additional shield, situated around the second conductor shield, separated by a second dielectric as well, though the third conductor could be a single wire insulated from the second conductor shield. The second conductor functions as a driven shield and is connected to the output of a unity gain amplifier, or more generally a transfer function of equal to or less than unity gain, whose input is connected to the center conductor. The ground reference is the third conductor.
(Note that the noninverting unity gain amplifier effectively has it's output and input coupled together through the capacitance in the audio cable. While technically a unity gain amplifier would oscillate under these conditions, in practicality a unity gain amplifier sees a loop gain slightly less than one due to imperfections in the system, such as conductor resistance and a finite amplifier output impedance, so that the system does not oscillate. Please note that while the term “unity gain” is used herein, it should always be understood that the loop gain must be less than one to ensure no oscillations will occur.)
The connectors on a reduced capacitance cable, which uses the driven shield technique, are in the prior art either a) two conductor connectors, which do not participate in the driven shield capacitance reduction happening along the length of the cable, thus adding some parasitic capacitance of their own, or b) three-conductor connectors which carry the driven shield through the connector, having their capacitance reduced, but exposing the driven shield signal to the outside world.
As an example of the former, Dunseath, Jr. (U.S. Pat. No. 4,751,471, Jun. 14, 1988) discloses a driven shield cable with a connector: “As shown in FIG. 2, the lead wire connector 9 is a miniature phone plug with the output signal and battery common (ground) connected to the plug.” The referenced connector is a standard prior art device and is not able to participate in the driven shield capacitance reduction technique applied to the cable.
An example of the latter three-conductor device is a standard triaxial cable connector, such as that marketed by Pomona Electronics as the model 5056 male connector. This connector has three conductors and can participate in the driven shield capacitance reduction technique. However, the driven shield signal is exposed to the outside world, and the connector is not designed to be connected to a mating two-conductor connector.
Adding a third shielding conductor to each connector on a cable allows the driven shield electronics to eliminate the capacitance of the connectors as well, reducing the capacitance of the entire cable assembly to just a few picofarads. Hiding the driven shield conductors at the mating surfaces of one or both connectors permits connection to preexisting two-conductor equipment while gaining the benefits of low capacitance cabling and connectors. This technique is not taught in the prior art. Additionally, applying the driven shield technique to connector as well as cable eliminates the interdependency of capacitance and connector geometry.